1. Field of the Invention
The present invention relates to modulating digital communications and more particularly to using Biphase or FM encoding to modulate infrared communications.
2. Problem to be Solved
An important consideration in infrared (IR) communication is that the Infrared Data Access (IRDA) standard modulation scheme is heavily dependent on the data pattern being modulated. It is therefore not suitable for synchronous communication which requires that the received signal always include enough information to derive the bit rate clock and which benefits from a fairly constant DC signal level. In comparison to the IRDA standard, pulse position modulation (PPM) is quite adequate for infrared communication with respect to the power dissipation and data independent DC level shift. Hence, quite often the technique of pulse position modulation, e.g., four position PPM, is adopted for infrared communication because it requires few pulses to send a given amount of data while it maintains a constant DC level, independent of the data pattern being sent.
Offsetting these advantages, is the fact that PPM suffers a number of disadvantages as a larger number of positions are used. One disadvantage is that an analog phase lock loop becomes a mandatory requirement to track the incoming data. Consequently, the cost of the whole system is increased. A second disadvantage is that the DC level of a PPM signal with a large number of bit positions may not be as constant as desired. On the other hand, PPM also suffers a number of disadvantages as fewer bit positions are used. One such disadvantage is that the power dissipation increases with fewer bit positions. Further, PPM may not keep as constant a DC level as Biphase or FM coding schemes, even when only a few bit positions are used. Therefore, PPM encoding can be problematical with regard to synchronous communication in view of its DC level instability.
Biphase or FM encoding, involving biphase mark (FM1) and biphase space (FM0) encoding, and including a Manchester version, is a common modulation scheme in digital communication. The benefits of this encoding include the fact that the bit rate clock is carried as a part of the modulation and that it has a constant DC level which provides a balanced analog signal making it suitable for synchronous communication. However, in adapting it to infrared communication, this type of modulation is quite expensive in power dissipation in transmitting the signal. Nevertheless, it does have several attractive features. In addition to maintaining a constant DC level independent of the data pattern being transmitted, it also maintains the carrier frequency in a tight bandwidth. In comparison to PPM, the Biphase encoding scheme maintains a better constant DC level than four position PPM and it carries the bitrate clock as part of the encoded data while maintaining similar average power dissipation as four position PPM. Still, it is a drawback that Biphase or FM encoding typically consumes more power during transmission of an IR signal as compared to the IRDA and PPM modulation schemes. This excessive power consumption requirement during transmission of FM coding patterns via IR signal is the biggest concern regarding FM coding of infrared communications.
It accordingly is a problem in the art to find an infrared encoding or modulation system that is optimum in terms of power efficency while it maintains the constant DC level and clock information carrying traits of Biphase encoding.
Objects
It is therefore an object of the present invention to provide an efficient system and encoding method for use in infrared communication that also offers versatility.
It is a further object of the invention to provide an efficient system and encoding method using Biphase encoding in infrared communication.
It is another object of the invention to resolve the problems of the prior art by combining Biphase encoding with flash modulation in infrared communication.